The long-term goal of our research is to understand the biochemistry and cell biology of protein folding in eukaryotic cells. The proposed research will focus on folding events as they occur at the ribosome during synthesis of a polypeptide and will examine the role of molecular chaperones in the folding process. The conceptual framework required to understand the folding of proteins as they emerge from the ribosome originates from our previous work, which indicates that folding in the eukaryotic cytosol is mediated by a chaperone network that is physically and functionally linked to translation. The objective of this proposal is to elucidate the mechanism by which chaperones mediate the folding of newly synthesized proteins in eukaryotic cells. To gain insight into this process we identified critical questions that will allow us to understand de novo folding, namely: (i) how and when do chaperones contact the emerging nascent chains?;(ii) what is the relevance of chaperone-binding for de novo folding?;(iii) what is the contribution of different chaperone systems to overall folding in the cell? (iv) how do chaperones interact with the translation machinery? Our general strategy to answer these questions is to combine in vitro and in vivo approaches to obtain mechanistic and functional insights into the role of chaperones in cellular folding. The first two specific aims will analyze the folding of model proteins in vivo and in cell-free translation lysates that faithfully represent the intact cytosol. Since our in vivo analysis indicates that different proteins exhibit distinct chaperone requirements, the third specific aim will examine the contribution of different chaperones to cellular folding and will identify the substrate spectrum of different molecular chaperones. Finally, our fourth specific aim will explore the interaction between chaperones and the translational machinery.